1. Field of the Invention
The present invention relates to a position detecting method and apparatus in a position detector such as a digitizer or tablet and, more particularly, to an improvement in a method and apparatus for scanning many sensor coils provided side by side on a sensor unit of the position detector.
2. Description of Related Art
A variety of position detecting methods in position detectors have been known. For example, U.S. Pat. No. 5,466,896, the disclosure of which is hereby incorporated by reference, and Japanese published patent application No. 3-147012 disclose an electromagnetic transfer system. FIG. 7 is a schematic block diagram illustrating the basic operation of a position detector employing an electromagnetic transfer system.
A position indicator 110 shown in FIG. 7 has a resonance circuit 101 composed of a coil and a capacitor. A position detector 111 is constituted by a sensor unit 102, a coil selector 103, a send/receive selector 104, a high-frequency signal generator 105, a receiving circuit 106, and a signal processing unit 107. The position indicator 110 preferably has a pen shape or a puck shape. The pen shape suggests a writing tool that indicates a position when it is held in a hand of an operator. The puck shape suggests a cursor that continues to indicate a position even when the operator releases it, i.e. it can be left alone. The operator holds the position indicator 110 to specify a position (pointing entry), enter hand-written characters, drawing, or to perform other coordinate input operation on a computer. The position indicator 110 includes at least a coil or the resonance circuit 101.
The sensor unit 102 has a plate-like sensor surface, a point on which is defined by an X-Y rectangular coordinate. The sensor unit 102 is formed by arranging many sensor coils 100 side by side in an X-axis direction 108 and a Y-axis direction 109. In FIG. 7, only the sensor coils arranged in the X-axis direction are shown in order to avoid confusion. There are actually a few tens of the sensor coils; however, only three sensor coils are illustrated in the drawing for clarity.
In the position detector 111 which employs the electromagnetic transfer system, electromagnetic waves are transferred between the sensor coils 100 on the sensor unit 102 and the position indicator 110 in order to determine the coordinate value of a position specified by the position indicator according to a signal received by the sensor coils 100. Further, the position detector 111 is preferably provided with means for entering information from the switch of the position indicator or information on writing pressure in addition to the information on the coordinate value of the position indicator.
Two sets of sensor units are normally provided. The two sets of coils are preferably orthogonalized in the X-axis direction and the Y-axis direction, respectively, in order to perform coordinate detection in two directions, namely, the X-axis direction and the Y-axis direction. In this case, it should be noted that "the sensor coils in the X-axis direction" means that they are arranged in the X-axis direction rather than meaning that they are extended in the X-axis direction. As illustrated in FIG. 7, the lengthwise direction of the sensor coils in the X-axis direction agrees with the Y-axis direction.
The high-frequency signal generator 105 is a circuit for generating an AC signal of a predetermined frequency (e.g. a predetermined value in a range from a few hundreds of kilohertz to a few megahertz). The receiving circuit 106 is a circuit composed primarily of an amplifier. The signal processing unit 107 is composed primarily of a processor (CPU) and a storage circuit; it carries out XY-coordinate calculation according to the output of the receiving circuit 106. The processor of the signal processing unit 107 functions to control the coil switching of the coil selector 103 and the switching of the send/receive selector 104. For the purpose of clarity, the signal lines for controlling the coil selector 103 and the signal lines for controlling the send/receive selector 104 are omitted in FIG. 7. The coil selector 103 may be constituted by a well-known multiplexer. Likewise, the send/receive selector 104 may be constituted by a well-known switching circuit.
The position detecting process in the electromagnetic transfer system will now be described. It is assumed that the foregoing processor of the signal processing unit 107 has set the send/receive selector 104 for the send mode, namely, for the high-frequency signal generator 105, and the processor has set the coil selector 103 to select one particular sensor coil 100 in a sensor coil group of the sensor unit 102.
The high-frequency signal generator 105 generates a high-frequency signal and applies that signal to the selected sensor coil 100 via the send/receive selector 104 and the coil selector 103. The sensor coil 100 then produces an electromagnetic wave that will be referred to as "transmitter signal." When the position indicator 110 is placed near the sensor surface under this condition, the resonance circuit 101 in the position indicator 110 resonates due to the transmitter signal. Then, the processor of the signal processing unit 107 sets the send/receive selector 104 to the receive mode, namely, for the receiving circuit 106, to stop the issuance of the transmitter signal from the sensor coil 100. In other words, the supply of the high-frequency signal from the high-frequency signal generator 105 is stopped.
Under this condition, the oscillatory phenomenon in the resonance circuit 101 incorporated in the position indicator 110 does not stop immediately; damping oscillation continues for a while. Hence, the coil of the resonance circuit 101 generates an electromagnetic wave that will be referred to as a "response electromagnetic wave." The sensor coil 100 receives this response electromagnetic wave, and the signal received by the sensor coil 100 at this time will be referred to as a "received signal". The received signal is sent to the receiving circuit 106, where it is processed, via the coil selector 103 and the send/receive selector 104. The signal which has been processed by the receiving circuit 106 is further handed to the signal processing unit 107 which performs XY coordinate calculation and the analysis of switch information according to the amplitude, phase, and so on of the processed signal. The obtained coordinate value and switch information are sent out to a host apparatus not shown, i.e. an external computer.
The resonance circuit 101 in the position indicator 110 shown in FIG. 7 is represented as a coil or the resonance circuit 101 because the resonance phenomenon is not necessary as long as magnetic coupling takes place between the sensor coils.
The sending and receiving operation of the sensor coils 100 is repeated while switching in sequence among the multiple sensor coils 100 on the sensor unit 102 in a position detecting direction. The operation of switching among the multiple sensor coils 100 in sequence will be referred to as "scanning".
It has already been mentioned that, among the components making up the position detector 111, the coil selector 103 composed mainly of a multiplexer is primarily responsible for selecting and switching among the multiple sensor coils 100. Also, it has already been mentioned that the coil selector 103 is connected to the processor of the signal processing unit 107 by a signal line, which is not shown, and it is controlled by the processor. The program describing the operation of the processor is stored in a storage device called a ROM (read-only memory) of the signal processing unit 107. The storage device is preferably a component of the position detector. The processor reads the program stored in the ROM and executes the scanning according to the program. Accordingly, the position detecting process, particularly the scanning method for the sensor coils 100, can be modified by the program stored in the ROM.
The position detecting process will now be described.
The position detecting process includes the procedure from a point at which no coordinate information (not even an approximate position) on the position indicator 110 has been obtained to a point at which the detailed coordinate of the position indicator is calculated. As previously mentioned, this process corresponds to the processing contents of the processor of the signal processing unit 107 shown in FIG. 7. From this viewpoint, the position detecting process is not merely scanning (selecting in a predetermined sequence) the sensor coils 100. The position detecting process also includes processing in which the processor of the signal processing unit 107 acquires the output obtained by the receiving circuit 106 as the result of the selection, the processing for carrying out the coordinate calculation based on the signal level obtained from the previous processing, and the processing for sending out the coordinate value, which has been finally obtained, to external equipment, i.e. a computer which is host equipment in most cases.
The processing for obtaining the output signal of the receiving circuit 106 is implemented immediately after every scan of each of the sensor coils 100. The coordinate calculation processing is implemented immediately after the receiving levels (the voltage levels of the transmitter signals of the receiving circuit 106) at a plurality of (about 2 to about 4) sensor coils located in the vicinity of the position indicator 110 have been obtained. As a specific method for the coordinate calculation, 2-point technique or 3-point technique (quadric function approximation) is known. The coordinate value sending-out processing is implemented upon completion of the coordinate calculation. The obtained coordinate value is sent out to external equipment by using a well-known interface circuit such as a means which conforms to the RS-232C standard.
This description will now focus on the scanning procedure of the sensor coils 100, abstracting the procedure for obtaining the output of the receiving circuit 106, the coordinate calculation processing, and the processing for sending out the coordinate value.
FIG. 8 shows a flowchart of the position detecting process in a conventional electromagnetic transfer system. As illustrated in FIG. 8, the process for detecting a position of the position indicator may be roughly divided into an all-scan process and a sector-scan process. The term "all-scan" indicates scanning the sensor coils over the entire area of the sensor coil surface, i.e. the surface on which the X- and Y-axis sensor coil is provided side by side. All-scan does not always refer to a case where all sensor coils are scanned; it may refer to a case where, for example, every other sensor coil is scanned. In the flowchart, only "scan" is shown for the purpose of simplicity; the actual scanning operation, however, includes a plurality of steps. For this reason, the boxes with double-line sides are used for steps ST200 and ST400 of FIG. 8. In the actual operation, every time each sensor coil 100 is scanned, the control step for switching the send/receive selector 104 and the processing step for acquiring the transmitter signal of the receiving circuit 106 are carried out.
The processing for detecting a position indicator starts with the all-scan process (step ST200). The all-scan process is implemented for both the X-axis and the Y-axis. This all-scan may be regarded as rough detection because it is intended mainly for quickly obtaining the approximate position of the position indicator 110.
At the completion of the all-scan process, the processor will have obtained the signal intensity distribution of the received signals on the sensor unit according to the signals received from the sensor coils 100. This is illustrated in FIG. 9. The nearly square member in FIG. 9 represents the sensor unit 102. The thick arrows crossing on the sensor unit 102 indicate the coil selecting directions of the X-axis and Y-axis. As shown in FIG. 9, it is assumed that the position indicator 110 is pointing at a certain position on the sensor unit 102. The signal intensity distribution obtained by the processor upon completion of the all-scan process is shown by the bar graph shown in FIG. 9. The bar graph shows only the intensity distribution obtained by the scan in the X-axis direction. As indicated by the bar graph, when the position indicator 110 is located near the sensor surface, the intensity of the received signal of the sensor coil closest to the position indicator 110 shows the highest value. Hence, the group of several sensor coils around the sensor coil giving the highest value shows the peak of the signal intensity distribution. This makes it possible to know the approximate position.
FIG. 9 shows a constant level of signal intensity in the area other than the group of several sensor coils around the sensor coil giving the highest value. Such a constant level of signal intensity is sometimes called an offset value because there is a certain level of output even when no input is applied to the receiving circuit 106 in FIG. 7.
The program determines in step ST299 whether the signal intensity of the received signal shown in FIG. 9 is larger than a predetermined value. If the signal is smaller than the predetermined value, the program goes back to the all-scan step ST200. If the signal is larger than the predetermined value, the program moves onto the sector-scan step ST400. The predetermined value is a "threshold value" that is a preset appropriate value above the offset value mentioned above.
In the sector-scan step ST400, the foregoing sending and receiving operation is repeated using the sensor coil at the central position and several sensor coils adjacent thereto obtained as the result of the all-scan step. As shown by the coil selecting area in FIG. 10, the sector-scan step ST400 is carried out at least on the X-axis and the Y-axis. This allows the detailed signal intensity distribution to be obtained as shown in FIG. 10; the coordinate value is determined by performing the interpolative calculation of the respective received signals in the signal processing unit 107 shown in FIG. 7. Thus, the sector-scan is a more detailed detection process.
The term "more detailed" means more detailed both time-wise and space-wise. More specifically, "more detailed in time" comes from the fact that the time required for the sector-scan is shorter than that required for the all-scan. For instance, if all-scan takes five times as long as the sector-scan takes, then it means that five coordinate values can be obtained by the sector-scan in the same time as that for the all-scan to obtain one coordinate value. This characteristic contributes to good trackability primarily in the dynamic characteristic of the position detector, especially when the position indicator moves quickly.
The situation of "more detailed space-wise" may occur when the all-scan step skips some coils (the all-scan can include the skipping scan as previously mentioned). No skipping occurs during the sector-scan step. This is a problem with the spatial gaps of the sensor coils used; therefore, the spatial detailedness contributes to the resolution or accuracy of the obtained coordinate value.
As shown in FIG. 8, when the position detector 111 initially starts up, the program performs the all-scan in step ST200. If an approximate position of the position indicator 110 is found, the program gives an affirmative determination result (YES) in step ST299 and proceeds to the sector-scan in ST400. If no approximate position is found, the program gives a negative determination result (NO) in step ST299 and repeats the all-scan in step ST200 thereafter. The affirmative determination result (YES) is given in step ST499 as long as the coordinate of the position indicator 110 is obtained by the sector-scan; hence, the sector-scan is repeated. If the position indicator 110 is missed during the sector-scan, then the program gives the negative determination result (NO) in step ST499, so that it goes back to the all-scan in ST200. Thus, the two different scanning methods, namely, the all-scan and the sector-scan, are combined to accomplish efficient coordinate detection.
An example of the method for detecting a plurality of position indicators will now be described.
The method for detecting a single position indicator is called "single-scan"; the method for detecting a plurality of position indicators is called "multi-scan".
The same number of resonance circuits of different resonance frequencies as the number of position indicators to be detected are prepared, and all the position indicators are provided with the prepared resonance circuits. A position detector sends and receiving at particular frequencies for the position indicators and carries out all-scan or sector-scan for the respective resonance frequencies alternately so as to detect the multiple position indicators.
To multi-scan for two indicators, two position indicators having different, fixed resonance frequencies are prepared to enable a position detector to communicate with the position indicators by using the different frequencies. In this case, one of the following different scanning processes is selectively implemented: in a first process, all-scan is carried out for two different frequencies alternately; in a second process, two types of scanning are alternately carried out, namely, sector-scan for one frequency and all-scan for the other frequency; and in a third process, the sector-scan is carried out for the two frequencies alternately.
The description given above is based on the assumption that the electromagnetic transfer system is employed. There are, however, other position detecting systems. As a simple electromagnetic system, there is one in which electromagnetic waves are transmitted from a sensor surface and received by a position indicator, or electromagnetic waves are transmitted from the position indicator and received by the sensor surface. There is a cross type detection system in which a sensor coil in the X-axis direction transmits a signal and a sensor coil in the Y-axis direction receives it. There is also a self-oscillation type detection system disclosed in Japanese Unexamined Patent Publication No. 5-241722.
Thus, all the systems described above have, in common, a plurality of coils arranged side by side in the X and Y directions to scan the coils so as to perform coordinate detection. For this reason, all the systems are facing the task of achieving efficient scanning to detect a plurality of position indicators.
There has been, however, a problem with the multi-scan for detecting the positions of a plurality of position indicators at the same time. Detecting the positions of the position indicators requires that the sensor coils of the position detector be scanned alternately for the same number of times as the number of position indicators. In comparison with the single-scan for detecting the position of only one position indicator, the multi-scan avoidably exhibits deteriorated performance in the trackability of coordinate values in relation to the actual positions of the position indicators when the position indicators are moved, or a deteriorated dynamic characteristic which is the performance involved in relatively quick moving of position indicators.
Normally, the trackability of the multi-scan for detecting two position indicators is reduced simply to half of that of the single-scan for detecting only one position indicator, thus leading to deterioration in performance of the multi-scan.
More specifically, in the case of the multi-scan for detecting at least two position indicators, the sensor coils of the position detector are scanned alternately to detect the position indicators. This has resulted in slower recognition of the position indicators.